As is well known in the art, the fabrication of a typical input pinion for an automotive differential assembly is complex and costly and as such, greatly adds to the cost of the differential assembly. A typical process for forming an input pinion includes forging of the gear and shaft, annealing, straightening, rough machining, carburizing, hardening and finish machining operations. Additionally, the input pinion may be processed through a lapping operation wherein the teeth of the input pinion are lapped with the teeth of the ring gear to match the input pinion and ring gear to one another. The lapping operation matches the input pinion and the ring gear to one another, necessitating that they be installed as a set to a given differential assembly. Despite the almost universal use of such forming processes, several drawbacks have been noted.
One such drawback relates to the initial forming of the input pinion through forging. As those skilled in the art will appreciate, the input pinion is typically blanked or rough-formed in a forging operation from a solid billet of steel. This forging operation is relatively complex due to the shape of the input pinion and as such, the tooling for the forging operation is generally complex and expensive.
Another such drawback concerns the machining of the input pinion. The numerous machining operations that are performed on the input pinion typically account for more 70% of the total cost of the input pinion. Furthermore, the protracted nature of the machining operations often results in an average cycle time that exceeds one or more days in length.
Yet another drawback concerns the material from which the input pinion is formed. Typically, the steel billet from which the input pinion is formed is a low carbon steel having characteristics that are particularly well suited to both forging and machining. Such steels, however, generally lack the strength that is desired for an input pinion and as such, a time consuming and costly carburization process is typically employed to create a layer of relatively high carbon steel on the surface of the input pinion. Carburization usually entails the placement of semi-finished input shafts into a heated, high-carbon environment for an extended period of time to permit carbon to migrate into the input shaft to a predetermined depth. The input pinion is subsequently heat treated so that the high carbon layer provides a level of strength and durability that is commensurate with the intended application.
Accordingly, there remains a need in the art for an improved input pinion manufacturing method that permits increased flexibility in the design of the input pinion that facilitates application specific customization and the adaptation of lower cost processes for its manufacture.